The proposed research will determine whether variations in the gene for melanopsin are correlated with SAD. Melanopsin, a photopigment found in retinal cells projecting to the circadian clock and other brain areas, is implicated in non-visual responses to environmental light. Etiological models and treatment outcome research for SAD suggest that winter-time disruption of retinal light detection may underlie the pathology of SAD. If variations in melanopsin segregate with SAD, melanopsin and retinal light detection will be implicated in the pathology of SAD. Dispositional dysregulation of responsiveness to light cues may provide further research opportunities regarding the significance of melanopsin variants; the interplay between melanopsin, non-visual light detection, circadian rhythms, and SAD; and bio-behavioral issues in treatment development such as mechanistic factors behind the efficacy of bright light therapy. Specific aims are: (1) to determine if individuals with SAD have a higher prevalence of the identified variant in melanopsin compared to healthy volunteers, and (2) to determine if the melanopsin variant segregates with SAD diagnosis in family members.